Adhesives for attaching wire network to photovoltaic cells

ABSTRACT

Provided are novel methods of fabricating photovoltaic modules using pressure sensitive adhesives (PSA) to secure wire networks of interconnect assemblies to one or both surfaces of photovoltaic cells. A PSA having suitable characteristics is provided near the interface between the wire network and the cell&#39;s surface. It may be provided together as part of the interconnect assembly or as a separate component. The interconnect assembly may also include a liner, which may remain as a part of the module or may be removed later. The PSA may be distributed in a void-free manner by applying some heat and/or pressure. The PSA may then be cured by, for example, exposing it to UV radiation to increase its mechanical stability at high temperatures, in particular at a, for example the maximum, operating temperature of the photovoltaic module. For example, the modulus of the PSA may be substantially increased during this curing operation.

BACKGROUND

In the drive for renewable sources of energy, photovoltaic technologyhas assumed a preeminent position as a cheap and renewable source ofclean energy. For example, photovoltaic cells using a Copper IndiumGallium Diselenide (CIGS) absorber layer offer great promise forthin-film photovoltaic cells having high efficiency and low cost. Ofcomparable importance to the technology used to fabricate thin-filmcells themselves is the technology used to collect electrical currentfrom the cells and to interconnect one photovoltaic cell to another toform a photovoltaic module.

Just as the efficiency of thin-film photovoltaic cells is affected byparasitic series resistances, photovoltaic modules fabricated frommultiple cells are also impacted by parasitic series resistances andother factors caused by electrical connections to the absorber layer andother electrical connections within the modules. A significant challengeis the development of current collection and interconnection structuresthat improve the overall performance of the module. Moreover, thereliability of photovoltaic modules is equally important as itdetermines their useful life, cost effectiveness, and viability asreliable alternative sources of energy.

SUMMARY

Provided are novel methods of fabricating photovoltaic modules usingpressure sensitive adhesives (PSAs) to secure wire networks ofinterconnect assemblies to one or both surfaces of photovoltaic cells. APSA having suitable characteristics is provided near the interfacebetween the wire network and the cell's surface. It may be providedtogether as part of the interconnect assembly or as a separatecomponent. The interconnect assembly may also include a liner, which mayremain as a part of the module or may be removed later. The PSA may bedistributed in a void-free manner by applying some heat and/or pressure.The PSA may be then cured by, for example, exposing it to Ultra Violet(UV) radiation to increase its mechanical stability at high temperaturesand, in particular, at the maximum operating temperature of thephotovoltaic module. For example, the modulus of the PSA may besubstantially increased during this curing operation.

In certain embodiments, a method of fabricating a photovoltaic moduleinvolves providing a photovoltaic cell comprising a surface, providingan interconnect wire network assembly having a conductive wire network,establishing an electrical contact between a portion of the conductivewire network and the surface of the photovoltaic cell, and stabilizingthis with a pressure sensitive adhesive (PSA). The PSA is providedadjacent to an interface between the portion of the conductive wirenetwork and the surface to provide mechanical support to the wirenetwork with respect to the surface of the photovoltaic cell and tomaintain the electrical contact. The method may also involve curing thePSA to increase its mechanical stability at an operating temperature ofthe photovoltaic module. In certain embodiments, curing involvesexposing the PSA to a dose of UV radiation. In other embodiments, curinginvolves exposing the PSA to at least about 145° C. to increasecross-linking of the PSA. The PSA may include one or more of thefollowing materials: a UV-reactive styrenic block copolymer, a cationiccuring epoxy-functional liquid rubber, a saturated polyacrylate, anacrylate monomer, and an acrylate oligomer, and an acrylated polyester.The PSA may be provided as a part of the interconnect wire assemblycomprises. In certain embodiments, an interconnect wire assemblyincludes a liner. The liner may be removed after curing the PSA.

A PSA may be used for attaching a conductive wire network to a frontlight incident surface of the photovoltaic cell or its back sidesurface. A wire network may include one or more wires having a gauge ofbetween about 34 and 46. Establishing an electrical contact between theportion of the conductive wire network and the surface of thephotovoltaic cell may involve passing a pre-aligned stack of thephotovoltaic cells and the interconnect wire network assembly through aset of heated nip rollers. In the same or other embodiments,establishing an electrical contact between the portion of the conductivewire network and the surface of the photovoltaic cell involves heatingthe PSA to at least about 80° C. to allow the PSA to flow adjacent tothe interface between the portion of the conductive wire network and thesurface.

In certain embodiments, a PSA includes individual structures forming apattern corresponding to individual wires of the conductive wirenetwork. These individual structures do not completely extend over thesurface of the photovoltaic cell in between the individual wires of theconductive wire network. For example, individual structures may beshells provided on wires of the wire network. PSA may provide initialmechanical support to a portion of the conductive wire network withrespect to the surface of the photovoltaic cell prior to curing the PSA.

In certain embodiments, establishing an electrical contact between theportion of the conductive wire network and the surface of thephotovoltaic cell involves forcing the interconnect wire networkassembly that contains the PSA against the surface of the cell. Thisforce may redistribute the PSA provided around individual wires of theconductive wire network. In certain embodiments, an operatingtemperature of the photovoltaic module corresponds to a predeterminedmaximum operating temperature. In the same or other embodiments, anoperating temperature of the photovoltaic module is at least about 120°C.

Provided also a method of fabricating a photovoltaic module. The methodmay involve providing a photovoltaic cell including a surface, providingan interconnect wire network assembly including a conductive wirenetwork, and establishing an electrical contact between a portion of theconductive wire network and the surface of the photovoltaic cell. Apressure sensitive adhesive (PSA) is redistributed adjacent to aninterface between the portion of the conductive wire network and thesurface and provides support to the portion of the conductive wirenetwork with respect to the surface. The PSA may be a non-Newtonian PSAor a thixotropic PSA provided as a part of the interconnect wire networkassembly. In these embodiments, establishing the electrical contactbetween the portion of the conductive wire network and the surface ofthe photovoltaic cell involves applying a pressure between theinterconnect wire network assembly and the photovoltaic cell toredistribute the thixotropic PSA adjacent to the interface. The PSA mayalso be a thermoplastic PSA provided as a part of the interconnect wirenetwork assembly. In these embodiments, establishing the electricalcontact between the portion of the conductive wire network and thesurface of the photovoltaic cell involves heating the PSA to at least apredetermined temperature to redistribute the thermoplastic PSA adjacentto the interface.

Provided also a photovoltaic module including a first photovoltaic cellhaving a front side surface, a second photovoltaic cell having a backside surface, a conductive wire network having a first portion and asecond portion. The first portion of the network is in electricalcontact with the front side surface of the first photovoltaic cell,while the second portion is in electrical contact with the back sidesurface of the second photovoltaic cell. The module also includes afirst PSA provided adjacent to a first interface between the firstportion of the conductive wire network and the front side surface of thefirst photovoltaic cell. The first PSA may have a sufficient mechanicalstability to support the first portion of the conductive wire networkwith respect to the front side surface of the first photovoltaic cell atan operating temperature of the photovoltaic module. In certainembodiments, the module also includes a second PSA provided adjacent toa second interface between the second portion of the conductive wirenetwork and the back side surface of the second photovoltaic cell,wherein the second PSA has a sufficient mechanical stability to supportthe second portion of the conductive wire network with respect to theback side surface of the second photovoltaic cell at an operatingtemperature of the photovoltaic module.

These and other features are described further below with reference tothe figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a photovoltaic module havingmultiple photovoltaic cells electrically interconnected usinginterconnect wire network assemblies, in accordance with certainembodiments.

FIG. 2 is a schematic side view of two photovoltaic cells interconnectedusing a wire network assembly, in accordance with certain embodiments.

FIGS. 3A and 3B are schematic cross-sectional views of an interconnectwire network assembly attached to a surface of the photovoltaic cell, inaccordance with certain embodiments.

FIG. 4 illustrates a process flowchart corresponding to a method offabricating a photovoltaic module, in accordance with certainembodiments.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Thepresent invention may be practiced without some or all of these specificdetails. In other instances, well known process operations have not beendescribed in detail so as to not unnecessarily obscure the presentinvention. While the invention will be described in conjunction with thespecific embodiments, it will be understood that it is not intended tolimit the invention to the embodiments.

Making electrical connections to the front and back surfaces of aphotovoltaic cell, for example a Copper Indium Gallium Diselenide (CIGS)cell, can be challenging. Not only do these electrical connections needto have a relatively low electrical resistance and meet various rigorousrequirements specific to photovoltaic modules (e.g., minimize lightshading of the front surface), but these connections also have towithstand harsh operating conditions over the entire operating lifetimeof the photovoltaic module. For example, photovoltaic modulescontinuously go thought temperature cycles during their operation (e.g.,between high temperatures during a hot, sunny day and low temperaturesduring a cold night). The temperature difference in a single day mayexceed more than 100° C. These differences may be even more over atypical year (e.g., adding seasonal temperature fluctuations) and overthe entire operating lifetime. The temperature fluctuation may befurther amplified by various designs of photovoltaic module. Forexample, some rigid modules may be supported using metal brackets andpositioned at a distance from the roof surface allowing for ventilationand cooling. Flexible and building integrable photovoltaic modules mayhave very little or no gap between back surfaces of the modules andsupporting structures and, therefore, may get even hotter during theday.

These electrical connections to the front and back surfaces may be madeusing interconnect wire network assemblies. The assemblies include wirenetworks, each wire network including one or more wires (e.g., aserpentine shaped wire or multiple substantially parallel wires). Aportion of the wire network is placed in direct contact with a frontsurface or a back surface of the cell during module fabrication. Theother portion of the wire network may be connected to another cell orother electrical components of the module. In certain embodiments, oneportion of the network is placed in direct contact with a front surfaceof one cell, and another portion of the same network is placed in directcontact with a back surface of another cell. In these embodiments, thisnetwork provides an in-series electrical connection between these twocells. Other types of connections are possible as well, such as parallelconnections or a combination of in-series and parallel connections. Incertain embodiments, an interconnect assembly acts as a currentcollector from a less conductive surface of the photovoltaic cell, suchas a front surface containing transparent conductive oxide.

A wire network may be supported with respect to a surface of thephotovoltaic cells using various polymer materials. For example, apolymer material may be provided adjacent to the interface between thewire network and the surface. The polymer material may be bonded to boththe wire network and surface by melting the material or having someinitial tackiness. However, this support has to be maintained during theentire operating lifetime of the photovoltaic module and be resistant toall temperature fluctuations experienced by the module, some of whichare highlighted above. At the same time, various module components mayhave very different coefficients of thermal expansion (CTEs) and maycause mechanical stresses during these temperature fluctuations. Some ofthese stresses may force wires of the wire network to separate from thesurface of the photovoltaic cell. Furthermore, some polymer materialsmay soften at elevated temperatures and effectively let the wire moveunder the stress. This phenomenon is sometimes referred to as “wirefloating.” Wire floating can be detrimental to cell performance bycausing losses of electrical connections and the degradation of cellperformance.

It has been found that wire floating may be substantially reduced, andmodule performance substantially improved, by using more robust polymermaterials to support wire networks with respect to the photovoltaiccells. Specifically, materials that have higher melting temperaturesand/or glass transition temperatures may be used. In certainembodiments, suitable materials have melting temperatures exceeding anoperating temperature of the module, for example the maximum operatingtemperature. In the same or other embodiments, the melting temperatureof suitable materials is at least about 90° C. or, more specifically, ofat least about 100° C. or even at least about 110-120° C.

While these materials may help to reduce the wire floating phenomena,materials that require high temperatures during initial application andlater processing may still be challenging to work with and expensive toprocess. For example, attachment of the wire network to a surface may beobtained by some redistribution of a polymer material used for bonding.This, in turn, may require significant heating of the polymer materialto substantially reduce its viscosity and allow it to flow in order forthe redistribution to occur in a substantially void-free manner. Voidsare highly undesirable in the photovoltaic modules and may causemechanical stresses and other problems in the module. Furthermore, if apolymer material is not capable of providing some initial bondingbetween the cell and the wire network prior to the redistributiondescribed above, then this heating may need to occur after these twocomponents are aligned. Many high temperature polymer materials are notsufficiently “tacky” at room temperature to bond to other materials.Suitable tackiness can be obtained by heating, and some heating may beused, particularly localized heating, heating to lower temperatures, andheating for short periods of time. But additional heating may complicateprocessing.

It has been found that mechanical bonds and electrical connections maybe established between various components of the interconnect wirenetwork assembly and photovoltaic cell without a need for complexheating schemes. Instead, a set of polymer materials has been identifiedand tested for photovoltaic applications. Some of these polymermaterials fall into a class of PSA. Some PSA materials have initialtackiness even at room temperature. Such materials may be used toprovide initial bonding of wire networks to cells without an immediateneed for heating and other specific processing techniques. Furthermore,PSA materials have particular non-Newtonian or thixotropiccharacteristics and become less viscous when a shear force is applied tothese materials. Such materials may flow under pressure applied duringmodule assembly without a need for excessive heating but remainrelatively stable even at high temperatures during module operation.

Finally, some PSA materials may be cured to change their thermal orrheological characteristics or, more specifically, their mechanicalstrength and stability at high temperatures. For example, a PSA materialhaving good room temperature tack may be initially applied near thewire-surface interface. Some heating may be provided during thisfabrication stage to improve wet-out and flow, but the temperature doesnot need to be high. For example, the PSA material may be heated to aprocessing temperature that is substantially lower than the maximumoperating temperature of the photovoltaic module. Yet, this processingtemperature may still sufficient for the PSA material to flow in asubstantially void free manner. The PSA material can then be then curedby exposing it to UV radiation, e-beam radiation, high temperature, orsome other curing conditions to change the thermal/rheologicalproperties of the PSA material. Specifically, the mechanical stabilityat an operating temperature of the photovoltaic module, for example themaximum operating temperature, may be substantially increased duringthis curing operation. This may correspond to an increase in modulus,shear resistance, and glass transition temperature of the PSA material.Without being restricted to any particular theory, some PSA materialsmay increase their cross-linking upon being subjected to various curingtechniques. More cross-linked polymers tend to have higher modulus/shearresistance and are generally more mechanically stable at an operatingtemperature of the photovoltaic module, for example the maximumoperating temperature.

To provide abettor understanding and context for methods of fabricatingphotovoltaic modules and various features of module components, such asinterconnect wire network assemblies and cell surfaces, some examples ofphotovoltaic modules in accordance with embodiments of the presentinvention will now be described in more detail. FIG. 1 is a schematictop view of photovoltaic module 100, in accordance with certainembodiments. Module 100 includes multiple (photovoltaic cells 104electrically interconnected using interconnect wire network assemblies106. Specifically, all cells 104 shown in FIG. 1 are electricallyinterconnected in series such that each cell pair has one interconnectassembly extending aver a front surface of one cell and extending undera back surface of another cell. Module 100 shown in FIG. 1 includeseight photovoltaic cells 104 that are interconnected using sevenassemblies 106. However, it will be understood by one having ordinaryskill in the art that any number of cells may be positioned within onemodule. In certain embodiments, a module has at least 10 cells or, morespecifically, at least 15 cells interconnected in series. In particularembodiments, a module has 22 cells interconnected in series.Furthermore, a module may have multiple interconnected sets of cellssuch that the sets are further connected with each other. For example, amodule may include two sets, with each set including 22 interconnectedcells.

Multiple cells or sets of cells may be interconnected in series toincrease an output voltage of the module, which may be driven by currenttransmission and other requirements. For example, a typical voltageoutput of an individual CIGS cell is between 0.4V and 0.7V. Modulesbuilt from CIGS cells are often designed to provide voltage outputs ofat least about 20V and even higher voltage ratings. In addition tointerconnecting multiple cells in series, a module may include one ormore module-integrated inverters to regulate its voltage output.Interconnect assemblies may be also used to connect multiple cells inparallel or various combinations of the two connection schemes (i.e.,parallel and in-series connection schemes).

Each interconnect assembly 106 illustrated in FIG. 1 includes aserpentine-shaped wire extending across the length of photovoltaic cell104 (direction X). Bottom portions (with respect to the moduleorientation in FIG. 1) of the serpentine-shaped wire extend under lowercells to make electrical connections to the back sides of these cells.These portions are illustrated with dashed lines. In certain embodimentsthese portions may also include conductive tabs welded to the wires inorder to increase the surface contact area with the back sides of thecells. A top portion of each wire is shown to extend over a front sideof a cell and making an electrical connection to the front side.

Most interconnect assemblies 106 extend both over a front side of onecell and under a back side of an adjacent lower cell. From aphotovoltaic cell perspective, most cells 104 have one interconnectassembly 106 extending over its front side and another extending underits back side. However, some end-cells (e.g., the top-most cell inFIG. 1) may have only one interconnect wire network assembly 106extending over one of their sides, typically over their front sides. Inthese embodiments, bus bars or other electrical components of the modulemay be electrically coupled directly to another side of such cells,typically their backsides. For example, FIG. 1 illustrates a portion oftop bus bar 108 extending under and connecting directly to the back sideof the top cell without any intermediate interconnect assemblies. Still,some end-cells (e.g., the bottom cell in FIG. 1) may be in contact withtwo interconnect wire network assemblies 106. A bottom bus bar 110 isshown electrically coupled to one of these assemblies 106 or, morespecifically, to the assembly 106 extending over the front side of thebottom cell. Bottom bus bar 110 may be electrically coupled to thisassembly 106 using a number of coupling techniques that are generallynot suitable for coupling to the cell, such as welding and soldering.

In certain embodiments, a front surface of the cell includes one or moretransparent conductive oxides (TCO), such as zinc oxide, aluminum-dopedzinc oxide (AZO), indium tin oxide (ITO), and gallium doped zinc oxide.The layer forming this surface is typically referred to as a topconductive layer or a top layer. A typical thickness of the topconductive layer is between about 100 nanometers to 1,000 nanometers or,more specifically, between about 200 nanometers and 800 nanometers, withother thicknesses within the scope. The top conductive layer provides anelectrical connection between the photovoltaic layer (positionedunderneath the top conductive layer) and portions of the interconnectassembly. Due to the limited conductivity of the top conductive layer,wires of the assembly typically extend over the entire front surface ofthe cell. Furthermore, the wires may be distributed substantiallyuniformly at least in the area overlapping with the front surface.

In the same or other embodiments, aback surface of the cell includes aconductive substrate supporting the photovoltaic layer as well ascollecting electrical current from this layer. Some examples of aphotovoltaic layer or stack include CIGS cells, cadmium-telluride(Cd—Te) cells, amorphous silicon (a-Si) cells, microcrystalline siliconcells, crystalline silicon (c-Si) cells, gallium arsenide multi-junctioncells, light adsorbing dye cells, and organic polymer cells. However,other types of photovoltaic stacks may be used as well. Whileinterconnect assemblies generally do not make direct connections to thestack, various characteristics of the photovoltaic stack create specificrequirements for the design of the interconnect assemblies. Someexamples of conductive substrates include stainless steel foil, titaniumfoil, copper foil, aluminum foil, beryllium foil, a conductive oxidedeposited over a polymer film (e.g., polyamide), a metal layer depositedover a polymer film, and other conductive structures and materials. Incertain embodiments, a conductive substrate has a thickness of betweenabout 2 mils and 50 mils (e.g., about 10 mils), with other thicknessesalso within the scope. Generally, a substrate is sufficiently conductivesuch that a uniform distribution of an assembly's components (e.g.,wires) adjacent to the substrate is not needed.

As described above, portions of interconnect wire network assemblies areelectrically coupled to the front and/or back surfaces of thephotovoltaic cells. This coupling is typically provided by directmechanical contact between wires of the networks and the surfaces. Themechanical contact may be stabilized by bonding these surfaces and wiresto other components, such as adhesive materials provided as a part ofthe assemblies or as separate components. In this way, support isprovided for establishment and maintenance of the contact. For example,adhesive materials may be provided on a liner as a part of the assembly(as further described below with reference to FIGS. 2 and 3), as acoating on wires of the wire network, or as a coating over the wirenetwork after it establishes a contact with the photovoltaic cell.

FIG. 2 illustrates a schematic side view of a module portion 200 thatincludes two photovoltaic cells 202 and 204 electrically interconnectedusing an assembly 206, in accordance with certain embodiments. Cell 202includes a substrate layer 222 supporting a photovoltaic layer 221 and atop layer 222. Similarly, cell 204 includes a substrate layer 218supporting a photovoltaic layer 217 and a top layer 216. Assembly 206includes one or more wires forming a wire network 208, a bottom carrierstructure 212, and a top carrier structure 214. Top carrier structure214 attaches a portion of wire network 208 to top layer 216 of cell 204in order to make an electrical connection between these two componentsor, more specifically, between wires of network 208 and the front sidesurface of top layer 216. This connection may require a certain overlap(in direction Y) between wire network 208 and top layer 216, the extentof which generally depends on the electrical properties of top layer216.

Bottom carrier structure 212 attaches another portion of wire network208 to bottom substrate layer 222 of cell 202 in order to make anelectrical connection between these two components or, morespecifically, between wires of network 208 and the bottom surface ofsubstrate layer 222. Substrate layer 222 may have higher conductivitythan a corresponding top layer. As such, wire network 208 may not needto overlap as much with substrate layer 222 as with the top layer asdescribed above.

Carrier structures used for attaching interconnect assemblies tophotovoltaic cells may have various designs and configurations.Generally, a carrier structure includes at least some PSA material. In afully fabricated module, the PSA material may be arranged as acontinuous layer or multiple individual patches. Furthermore, in a fullyfabricated module, the carrier structure may include a liner. An examplea module with a continuous PSA layer and a liner is further describedbelow with reference to FIG. 3. In other embodiments, a liner may beinitially provided as a carrier for the PSA material but them removed.Finally, a PSA material may be provided without a liner, for example, asa wire coating or as a melt (or as a solution) deposited over anarrangement including the wire network that is in contact with a surfaceof the photovoltaic cell.

FIG. 3 is a schematic cross-sectional view of a wire network 306attached to a surface 302 of photovoltaic cell 301 using a carrierstructure 307, in accordance with certain embodiments. Surface 302 mayrepresent either the front side surface (i.e., the light-incidentsurface) or the back side surface (i.e., the substrate surface).Depending on the type of the surface, wire network 306 and portions ofcarrier structure 307 will contact different types of materials, such astransparent conductive oxide or metallic substrate. In certainembodiments, the materials of the carrier structure 307 may bespecifically tailored to the requirements of the surface 302.

Carrier structure 307 includes a PSA layer 304 and a liner 308. Liner308 may be used as a temporary carrier for PSA layer 304 and may beremoved in later operations. Such liners are referred to as “releaseliners” and generally may be made from any type of material suitable forcarrying and releasing PSA materials. For example, a liner may be a thinpolymer film, a metal foil, or any other suitable material.Alternatively, liner 308 may remain as a part of the fully fabricatedmodule. In these embodiments, liner 308 is generally made fromelectrically insulating materials. When such liners are used over frontlight-incident sides of cells, the liners should also be substantiallytransparent to allow sunlight to reach the photovoltaic layer. Someexamples of suitable liner materials include thermoplastic materials,such as polyethylene terephthalate (PET), polyethylene naphthalate PEN,polyethylene-co-tetrafluoroethylene (ETFE), polyamide, polyetherimide(PEI), polyetheretherketone (PEEK), or combinations of these.

PSA layer 304 may be a continuous layer, as shown in FIG. 3A, or acollection of individual patches, as shown in FIG. 3B. The individualpatches may be positioned adjacent to the wire-surface interfaces 309but may not fully cover the surface of the cell in between the wires.The patches provided on a carrier liner should align with individualwires of the wire network to ensure adhesion of these wires to thecell's surface. A patched PSA layer is typically present in the finalmodule assembly without a corresponding liner to prevent voids inbetween the liner and the cell's surface in the areas where the PSApatches are not present. In certain embodiments, patches of PSA arecombined with patches of some other material to form a continuous layer,which may be used together with a liner.

PSA materials used for fabrication of a photovoltaic module may havespecific initial properties that assist with distribution of thesematerials in a void free manner during some initial operations of thefabrication process. In certain embodiments, some initial properties arechanged during fabrication to improve the bond between the wire networkand the cell's surfaces. Some materials that are capable of changingtheir properties include “dual stage” PSA materials. For example, somedual stage PSA materials increase their cross-linking between polymerchains upon being subjected to certain curing techniques. These changesmay be evident from an increase in modulus, shear resistance and/orglass transition temperatures. Specifically, some PSA materials may becured using UV radiation. Examples of such PSA materials include aUV-reactive styrenic block copolymer, a cationic curing epoxy-functionalliquid rubber, a saturated polyacrylate, and an acrylated polyester. Inother embodiments, some PSA materials may be cured by being exposed to ahigh temperature for a relatively short period of time. Examples of suchmaterials include peroxide cured acrylates. In yet other embodiments,some PSA materials have specific thixotropic characteristics, such thatthey become less viscous and can flow at room temperature (or someslightly elevated temperature) upon applying a certain shear force. Yetthese materials remain mechanically stable at much higher temperatureswhen no or lower shear forces are applied. Examples of such materialsinclude acrylate based PSA's supplied by Adhesive Research in Glen Rock,Pa. and MACtac Global in Stow, Ohio.

An initial thickness of PSA layer 304 is generally comparable to across-sectional dimension of the wires in wire network 306 (e.g., adiameter of the round wires or a thickness of the flat wires). Incertain embodiments, the initial thickness is between about 25% and 100%of the cross-sectional dimension of the wires or, more specifically,about 50%. Various examples of wires that may be used for wire network306 and their respective dimensions are described below. It should benoted that initially provided PSA materials may change their shapeduring fabrication. Therefore, the initial layer thickness may change.Furthermore, PSA materials may be provided in shapes other than a layerand later redistributed into a layer shape.

Wire network 306 may include one or more wires that are uniformlydistributed within a predetermined wire boundary. For example, eachnetwork may include one serpentine-shaped wire (as shown in FIG. 1) ormultiple parallel wires spaced apart along direction X. Arrangements ofone or more wires in the network may be characterized by a pitch 310,which, for purposes of this document, is defined as a distance betweenthe centers of two adjacent wires or two adjacent portions of the samewire. The pitch 310 determines the distance electrical current travelsthrough the surface layers of the cells prior to reaching the conductivewires. Reducing the pitch increases the current collectioncharacteristics of the interconnect assembly. However, a smaller pitchalso decreases the useful front surface area of the cell by covering thephotovoltaic layer with non-transparent wires and causes more densetopography, which may be prone to voids and other imperfections. Incertain embodiments, pitch 310 is between about 2 millimeters and 5millimeters (e.g., about 3.25 millimeters), though other distances maybe used, as appropriate.

Wires of wire network 306 are typically made from thin, highlyconductive metal stock and may have round, flat, and other shapes.Examples of wire materials include copper, aluminum, nickel, chrome, oralloys thereof. In some embodiments, a nickel coated copper wire isused. In certain embodiments, the wire is 24 to 56 gauge, or inparticular embodiments, 32 to 56 gauge (for example, 40 to 50 gauge). Inspecific embodiments, the wire has a gauge of 34, 36, 40, 42, 44, or 46.Additional wire examples are described in U.S. patent application Ser.No. 12/843,648, entitled “TEMPERATURE RESISTANT CURRENT COLLECTORS FORTHIN FILM PHOTOVOLTAIC CELLS,” filed Jul. 26, 2010, which isincorporated herein by reference in its entirety for purposes ofdescribing additional wire examples.

FIG. 4 illustrates a flowchart corresponding to a process 400 forfabricating a photovoltaic module, in accordance with certainembodiments. Process 400 may start with providing one or morephotovoltaic cells, in operation 402, and providing one or moreinterconnect wire network assemblies, in operation 404. Various examplesof photovoltaic cells and assemblies are described above. The providedinterconnect assembly includes at least a wire network. It may alsoinclude a PSA material and/or a liner. A PSA material may also beprovided in later operations. Operations 402 and 404 may be repeated(decision block 405) to provide additional photovoltaic cells and/orinterconnect assemblies. For example, all photovoltaic cells andinterconnect assemblies of the module may be aligned during theseinitial operations prior to establishing bonds between the cells andassemblies. In certain embodiments, a photovoltaic cell provided inoperation 402 may already be bonded to an interconnect assembly providedin operation 404. In later operations, this interconnect assembly may bebonded to another cell, and this cell may be bonded to anotherinterconnect assembly.

Process 400 may proceed with establishing an electrical contact betweena wire network of the interconnect assembly and a surface of thecorresponding photovoltaic cell, in operation 406. If a PSA material isprovided as apart of the assembly, then operation 406 may also involveredistributing some PSA material to allow the electrical contact tooccur. During this redistribution some PSA material can be positioned atthe wire-surface interface. For example, the PSA material (and perhapssome other components) may be heated to a predetermined temperature toallow the PSA material to flow. In certain embodiments, a meltingtemperature of the PSA provided in process 400 (i.e., prior to thecuring operation) is less than 80° C. or even less than 60° C. These PSAmaterials may be easily heated by heating the overall structure,including the cells and various components of the interconnect wirenetwork assembly. Alternatively, the heating may be localized by, forexample, passing an electrical current though wires of the wire networksand resistively heating the wires. The PSA material may be heated aboveits melting temperature during this operation. In the same or otherembodiments, the PSA is heated to at least about 80° C. or, morespecifically, to at least about 100° C. Other rheological and flowproperties can be used to characterize these materials as well.

In the same or other embodiments, an interconnect assembly may bepressed against the surface of the photovoltaic cell during operation406. This operation may be performed as a single-step operation using aheated roller or a two-step operation first using a cold roller forestablishing initial tack and then using a hot roller to flow thematerial. This pressure helps to remove the PSA material from thewire-surface interface (i.e., the “contact” interface) and make anelectrical contact between the wires and the surface. This pressure mayalso help the PSA material to fill the voids and/or form stronger bonds(e.g., achieve more “wetting”) between the PSA material and wires andbetween the PSA material and cell surface. In certain embodiments, afterredistribution of the PSA material, there are substantially no voidsbetween the PSA material and the wires and between the PSA materials andthe cell surface. If a liner is used, then redistribution of the PSAmaterial may also be designed to eliminate substantially all voids inbetween the liner and the cell surface.

In certain embodiments, operation 406 involves coating the cell surface,which has a wire network disposed over this surface, with a PSAmaterial. The PSA material may be provided in a molten form or adissolved form (e.g., a highly polar solvent). The PSA material may alsobe partially cured in operation 406. Additional curing may be providedin later operations. In certain embodiments, the PSA material has aninitial tackiness to provide some support to the wire network withrespect to the cell. Overall, after completion of operation 406, the PSAmay provide at least some initial mechanical support to the wire networkwith respect to the cell's surface regardless of how this initialsupport or bonds are established (tackiness, heating, pressure, or acombination of these methods).

Process 400 may continue with an optional operation 410 during which thePSA material is cured. The curing may change the internal structure andphysical characteristics of the PSA material; for example, the curingmay increase the PSA material's mechanical stability at a maximumoperating temperature of the photovoltaic cell. This may correspond toan increased modulus, shear resistance or other rheological changesand/or glass transition temperature, which may rise above a certainpredetermined level defined by the operating conditions of thephotovoltaic module.

One example of a curing technique is UV radiation. A dosage of UVradiation can be easily controlled in a product environment so that anadequate curing level is achieved. In certain embodiments, the dose isbetween 0.1 W/cm² and 6 W/cm², which depends on the formulation, thephoto-initiator used, and other factors. While some curing may bedesirable for various reasons as explained above, excessive crosslinking may turn the initial PSA into a brittle material that may not besuitable for photovoltaic applications. Another example of a curingtechnique involves exposure of the PSA material to an elevatedtemperature. For example, some PSA materials may be exposed to atemperature of at least about 145° C. to increase their polymercross-linking.

Process 400 may also involve removing a liner in an optional operation412. As stated above, a liner may be provided in one of the earlieroperations as a temporary carrier of the wire network and/or PSAmaterial. In certain designs, a liner may not be needed to provide finalsupport to the wire network. The PSA material may be designed to provideall the mechanical support needed after performing various operations asdescribed above.

CONCLUSION

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing the processes, systems and apparatus of the presentinvention. Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein.

What is claimed is:
 1. A method of fabricating a photovoltaic module,the method comprising: providing a photovoltaic cell comprising asurface; providing an interconnect wire network assembly comprising aconductive wire network; establishing an electrical contact between aportion of the conductive wire network and the surface of thephotovoltaic cell; stabilizing the electrical contact between a portionof the conductive wire network and the surface of the photovoltaic cellwith a pressure sensitive adhesive (PSA) provided adjacent to aninterface between the portion of the conductive wire network and thesurface to provide mechanical support to the wire network with respectto the surface of the photovoltaic cell and to maintain the electricalcontact; and curing the PSA to increase mechanical stability of the PSAat an operating temperature of the photovoltaic module, wherein the PSAcomprises individual structures forming a pattern corresponding toindividual wires of the conductive wire network and wherein theindividual structures do not completely extend over the surface of thephotovoltaic cell in between the individual wires of the conductive wirenetwork.
 2. The method of claim 1, wherein curing of the PSA comprisesexposing the PSA to a dose of UV radiation.
 3. The method of claim 1,wherein the PSA comprises one or more of the following materials: aUV-reactive styrenic block copolymer, a cationic curing epoxy-functionalliquid rubber, a saturated polyacrylate, an acrylate monomer, and anacrylate oligomer, and an acrylated polyester.
 4. The method of claim 1,wherein the provided interconnect wire assembly comprises the PSA. 5.The method of claim 4, wherein the provided interconnect wire assemblyfurther comprises a liner.
 6. The method of claim 5, further comprisingremoving the liner after curing the PSA.
 7. The method of claim 1,wherein curing comprises exposing the PSA to at least about 145° C. toincrease cross-linking of the PSA.
 8. The method of claim 1, wherein thesurface is a front light incident surface of the photovoltaic cell. 9.The method of claim 1, wherein the wire network comprises one or morewires having a gauge of between about 34 and
 46. 10. The method of claim1, wherein establishing an electrical contact between the portion of theconductive wire network and the surface of the photovoltaic cellcomprises passing a pre-aligned stack of the photovoltaic cells and theinterconnect wire network assembly through a set of heated nip rollers.11. The method of claim 1, wherein establishing an electrical contactbetween the portion of the conductive wire network and the surface ofthe photovoltaic cell comprises heating the PSA to at least about 80° C.to allow the PSA to flow adjacent to the interface between the portionof the conductive wire network and the surface.
 12. The method of claim1, wherein the PSA provides initial mechanical support to the portion ofthe conductive wire network with respect to the surface of thephotovoltaic cell prior to curing the PSA.
 13. The method of claim 1,wherein establishing the electrical contact between the portion of theconductive wire network and the surface of the photovoltaic cellcomprises forcing the interconnect wire network assembly comprising thePSA against the surface.
 14. The method of claim 13, wherein the PSA isredistributed around individual wires of the conductive wire networkduring establishing the electrical contact between the portion of theconductive wire network and the surface of the photovoltaic cellcomprises.
 15. The method of claim 1, wherein the operating temperatureof the photovoltaic module corresponds to a predetermined maximumoperating temperature.
 16. The method of claim 1, wherein the operatingtemperature of the photovoltaic module is at least about 120° C.
 17. Amethod of fabricating a photovoltaic module, the method comprising:providing a photovoltaic cell comprising a surface; providing aninterconnect wire network assembly comprising a conductive wire network;and establishing an electrical contact between a portion of theconductive wire network and the surface of the photovoltaic cell suchthat a PSA is redistributed adjacent to an interface between the portionof the conductive wire network and the surface and provides support tothe portion of the conductive wire network with respect to the surface,wherein the PSA comprises individual structures forming a patterncorresponding to individual wires of the conductive wire network andwherein the individual structures do not completely extend over thesurface of the photovoltaic cell in between the individual wires of theconductive wire network.
 18. The method of claim 17, wherein the PSA isa non-Newtonian PSA or a thixotropic PSA provided as a part of theinterconnect wire network assembly; and wherein establishing theelectrical contact between the portion of the conductive wire networkand the surface of the photovoltaic cell comprises applying a pressurebetween the interconnect wire network assembly and the photovoltaic cellto redistribute the thixotropic PSA adjacent to the interface.
 19. Themethod of claim 17, wherein the PSA is a thermoplastic PSA provided as apart of the interconnect wire network assembly; and wherein establishingthe electrical contact between the portion of the conductive wirenetwork and the surface of the photovoltaic cell comprises heating thePSA to at least a predetermined temperature to redistribute thethermoplastic PSA adjacent to the interface.